The present invention relates generally to gas turbine engines, and, more specifically, to airfoil manufacture therefor.
A primary objective in designing turbofan gas turbine engines for propelling aircraft in flight is weight reduction. The engine should have minimum weight consistent with adequate strength for durability and long life.
In large turbofan engines, the fan blades are correspondingly large and generate significant centrifugal force during operation. Disposed downstream from the blades are stationary guide vanes which are also relatively large. The blades and vanes have corresponding airfoils specifically configured to aerodynamically cooperate with ambient air channeled thereover for producing propulsion thrust during operation. Such airfoils are typically solid metal, which may be lightened in weight by introducing cavities therein, with an attendant manufacturing cost therefor.
Composite fan blades including laminations of structural fibers in a bonding matrix are also known. These composite fan blades enjoy reduced weight while maintaining high strength, yet are expensive to produce.
In another development, hybrid airfoils using a high strength metal, such as titanium, are formed with integral pockets therein for reducing weight while maintaining strength. And, a filler is bonded into the pockets for completing the required aerodynamic profile of the airfoil.
Such hybrid airfoils may be manufactured by initially forming or forging a nominal airfoil to approximate final shape, and then completing the final shape by machining. The individual pockets are machined into the forged airfoil and must be suitably configured for avoiding stress concentrations.
Since a fan blade operates under substantial centrifugal force, any discontinuities or abrupt changes in contour of the airfoil are local sites of stress concentration. Intervening ribs between adjacent filler pockets must be suitably sized with relatively large blending radii to reduce stress concentration in the blade, as well as reduce peeling stress on the filler as the blade strains during operation. Although weight is removed from the blade by providing the pockets and lightweight filler therein, the required separating ribs limit the amount of weight reduction practical.
Accordingly, it is desired to provide an improved method of forming a hybrid airfoil for reducing or eliminating the need for machined pockets for the filler, while maintaining airfoil strength.